Coming recently, as a measure against the environmental pollution and the atmospheric temperature rise, environmental projects relating to the combustion ratio are being worldwidely carried out. Accordingly, the automobile industries have been making efforts to reduce the weight of automobiles. In reducing the weight of automobiles, the automobile suspension spring is an important factor. However, the suspension spring has a considerable weight, and therefore it has become the object of the concern.
If the suspension spring is made to be light, the spring has to have a high stress capacity. The high stress capacity is achieved through the improvement of the fatigue strength and the sag resistance. If the fatigue strength is lowered, the fatigue life expectancy is shortened, thereby leading to an early breaking of the spring. If the sag resistance is small, the spring cannot support the automobile body, thereby leading to the contact of the automobile body with the bumper. Therefore, in order to overcome such problems, the sag resistance characteristics of the spring should be improved.
In accordance with such a trend, an Si added steel which has a superior sag resistance characteristics compared with the existing SAE6150 (Cr-V) alloy came to attract the attention of the industry. Of it, SAE9260 (1.8-2.2% Si, SUP7) has been developed into a superior material. However, it has the problems such as the shortening of the fatigue life expectancy due to the surface decarburization and the high cost for peeling the surface. In order to overcome such problems, SAE9254 was developed in which the Si content was reduced for preventing the surface decarburization without much aggravating the sag resistance, and in which Cr was added for reducing the decarburization. However, in view of the light weight of automobile which is presently emphasized, the sag resistance of SAE9254 is not satisfactory. Then a material (SRS60) having a more superior sag resistance property was proposed in Japanese Patent Publication No. Sho-57-27956, Sho-57-169062 and Sho-57-13148. This material is formed by adding a small amount of vanadium (V) into SAE9254 so as to improve the sag resistance property. This SRS60 achieved the improvements of the sag resistance property and strength, but it made no contribution to the improvement of the decarburization and the toughness.
Meanwhile, the stress capacity of springs has reached the limit owing to the development of the process and the improvement of the precision. Therefore the remaining task is to develop a high stress capacity spring material having a superior sag resistance capacity, a high elastic limit, a high fatigue strength and a high toughness even under a high stress.
In the case of the suspending spring, if the maximum design stress is improved from the conventional 110 kg/mm.sup.2 to 130 kg/mm.sup.2, the weight of the spring can be reduced by 25%.
Therefore, if the weight of spring is to be reduced, a spring material having a superior sag resistance characteristics is required. In this context, the addition of silicon is required, but in this case, the decarburization problem occurs during the hot rolling process and during the heat treatment. Further there is the problem that the high strength is accompanied by a low toughness. These problems have to be solved first of all.
The conventional techniques for inhibiting the decarburization are disclosed in Japanese Patent Laid-open No. Hei-2-301514, Hei-1-31960, Sho-63-216591, Sho-63-153240, Sho-58-67847, and Sho-58-27956.
According to Japanese Patent Laid-open No. Hei-2-301514 and Sho-63-153240, the content of chrome is increased to 1.5-3.0%, or lead, sulphur and calcium are added. However, the increase of the content of chrome lowers the sag resistance. Further, in the case of a similar alloy of Japanese Patent Laid-open No. Sho-62-274058, the content of silicon does not ensure the maximum level of the sag resistance characteristics.
Meanwhile, Sho-63-216591 and Sho-58-67847 propose that, the content of carbon be reduced, and copper, molybdenum, tin, antimony and arsenic be added. In this technique, there are the problems that the added elements are too expensive, and the toughness is decreased. Japanese Patent Laid-open No. Hei-1-31960 and Sho-58-27956 propose a lowering of the content of silicon. However, it cannot be expected that the decrease of the content of silicon can lead to the improvement of the sag resistance characteristics.
Meanwhile, high stress capacity materials are disclosed in Japanese Patent Laid-open No. Hei-3-2354, Hei1-184259 and Sho-62-170460. In Japanese Patent Laid-open No. Hei-3-2354, the content of carbon is reduced for improving the toughness, while molybdenum (Mo) and aluminum (A1) are added to improve the sag resistance characteristics through a grain refinement.
However, in the case of Japanese Patent Laid-open No. Hei-3-2354, the intended effect can be obtained through the distribution of the Mo precipitates. However, precipitating temperature of Mo is over 500.degree. C., and therefore, it is difficult to maintain the strengths of the matrix. The grain refinement can be expected by adding aluminum, but non-metallic inclusions of alumina series are formed, with the result that the fatigue characteristics are adversely affected, thereby making the technique undesirable.
Japanese Patent Laid-open No. Hei-1-184259 proposed a technique of attaining to high strengths by adding Mn, Cr, V, Ni, and Mo. However, in this case, during the increase of Cr (1.0-3.5%) and Mo, the considerable improvement of the hardenability causes to form the low temperature structure (bainite and martensite structures) during the manufacturing of the material. Therefore, during the manufacturing of springs, a difficulty is encountered in peeling the surface, with the result that a softening heat treatment has to be added.
Japanese Patent Laid-open No. Sho-62-170460 proposes that the formation of the non-metallic inclusions can be reduced by adding Ca, and that the sag resistance is improved through the grain refinement by adding Ti. However, in this case, the addition of Ti causes the formation of non-metallic inclusions of Ti series, with the result that the fatigue characteristics is aggravated.
Meanwhile, Japanese Patent Laid-open No. Hei-3-2354 proposes a technique for improving the toughness.
The above patent discloses techniques of improving the toughness by reducing carbon, and by adding nickel. However, in this case, the reduction of the content of carbon causes the lowering of the yield strength, with the result that the sag resistance is aggravated.